Depth from Defocus (DFD) indicates a method of measuring a distance, by using two or more images with different focus points, based on information on blurs. A captured blurred image is an image formed by convolving an all-in-focus image which represents a state of no lens blur with a point spread function which is a function for an object distance. Since the point spread function is a function involving the object distance as a variable, detecting a blur from the blurred images by the DFD can lead to the determination of the object distance.
Here, the all-in-focus image and the object distance are unknown quantities. For one blurred image, one equation involving the blurred image, the all-in-focus image, and the object distance holds. Capturing a new blurred image with a different in-focus position offers a new equation, and the obtained equations are solved to determine the object distance. How to obtain the equations, how to solve the equations, or the like is disclosed by Patent Literature (PTL) 1, Non Patent Literature (NPL) 1, and so on.